The present invention relates generally to antiozonants for adding to rubber formulations to inhibit or prevent the ozone cracking of the final rubber product. More specifically, the present invention relates to an organoclay-antiozonant complex, a method of preparing the organoclay-antiozonant complex, and a method of incorporating the organoclay-antiozonant complex into a natural or synthetic rubber formulation.
It is well recognized that both synthetic and natural rubbers undergo deterioration due to ozone, an energized triatomic molecular form of oxygen commonly found in the atmosphere. The deterioration of rubber on exposure to the atmosphere due to traces of ozone present in the atmosphere is different from any deterioration caused by oxidation due to oxygen found in the atmosphere and is generally not inhibited by the same compounds which may inhibit oxygen deterioration of the rubber. Ozone reacts with the rubber and causes cracking particularly in places where the rubber is under tension. It is believed that ozone attacks the double bond in diene compounds in the rubber to form a molozonide which is very unstable and subsequently splits apart to form a zwitterion and a ketone resulting in the cleavage of the polymer backbone of the rubber and the formation of cracks therein. Subsequent deterioration of the zwitterions can cause further cleavage of the polymer backbone of the rubber and accelerate crack propagation therein.
It is well known in the art to incorporate in rubber formulations certain additives, termed antiozonants, which inhibit and retard such ozone cracking. It is common to add such ozonants to rubber formulations used in producing tires and other industrial rubber products. Although certain petroleum waxes which bloom to the surface of the rubber compound have been moderately effective as antiozonants, such waxes cannot provide adequate surface protection in instances where the rubber is under tension due to bending or flexing. Accordingly, it has become customary in the production of rubber to add antiozonant chemicals directly to the rubber formulation. By far, the most common ozonants for adding directly to the rubber formulation are aromatic amines such as substituted paraphenylenediamines. The various substituted paraphenylenediamines suitable as antiozonants is vast. Examples of particular substituted paraphenylenediamines suitable as antiozonants in rubber formulations are given in U.S. Pat. Nos. 3,645,966; 3,663,505; 3,773,717; and 3,778,286.
Substituted paraphenylenediamines can be represented by the following general formula: ##STR1## There are three sub-categories of substituted paraphenylenediamines under this general formulation. If R and R' are both aryl compounds, then the substituted paraphenylenediamine is a diaryl-paraphenylenediamine. If both R and R' are both akyl hydrocarbons, then the substituted paraphenylenediamine is a dialkyl-paraphenylenediamine. If R is a alkyl hydrocarbon and R' a aryl hydrocarbon, or vice versa, then the substituted paraphenylenediamine is a alkyl-arylparaphenylenediamine.
A drawback of these antiozonant compounds is that they tend to be somewhat unstable in that they may react with other compounds added to the rubber formulation such as fillers, accelerators, softeners, extenders and waxes. Additionally, such antiozonant compounds are volatile and even soluble in water, both properties which lessen their effectiveness as antiozonants. Improved chemical stability within the rubber formulation, reduced volatility, and lower solubility in water would all enhance the effectiveness of antiozonant compounds.